1. Field of The Invention
The present invention relates to a flexible printed circuit and the method for making the same, especially to a printed circuit which can compensate the original characteristic impedance of the conductor trace thereof for obtaining a new value of characteristic impedance which is in an allowable range of a standard value.
2. The Prior Art
Flexible printed circuit (FPC) has been used in many fields for years such as the consumer products including the walk-man, the cellular phone, and the computer peripheral products including the printer, the disk drive. A conventional flexible printed circuit is shown in FIG. 7, wherein an adhesive layer 2 is formed in a flexible insulative board 1 and a pattern of conductor traces 3 are then formed on the adhesive layer 2 via a photoresist technique. This FPC works well only in low frequency but it may cause malfunction in high frequency. This is because the conductor trace 3 of the FPC functions as a transmission line other than a simple conductor line. The characteristic impedance of the conductor trace has to be considered in a high frequency environment. Normally, the characteristic impedance of a transmission line is determined by the line width, the line thickness, the dielectric constant of the insulative material, the distance thereof to the grounding potential.
Conventionally, to design an FPC having a desired constant characteristic impedance along the length thereof, an ideal grounding plane 4 such as copper foil shown in FIG. 8 is provided at an opposite side of the pattern of conductor traces 3 of FIG. 7. The characteristic impedance distribution is controlled by the physical structure of the transmission line such as the width and the thickness along the lengthwise direction thereof and the distance between the transmission line and the grounding plane. The physical dimension of the transmission line may have to be repeatedly adjusted along the lengthwise direction thereof according to repeated testing for maintaining the characteristic impedance thereof along the line within an allowable tolerance to a standard value. However, the job for repeatedly testing and adjusting the physical dimension of the transmission line is cumbersome, expensive, and may cause errors. It is even more difficult to adjust the distance between the transmission line and the grounding plane for obtaining a constant characteristic impedance along the line because this method needs to change the thickness of the insulative board repeatedly by sections.
It is requisite to provide a new structure of an FPC having a constant characteristic impedance value along a length thereof.